Gonadal steroid hormones are essential for mating in male hamsters, as in most mammals. Castration also reduces sexual motivation, as measured by interest in vaginal odors and preference for a receptive female. However, understanding of mechanisms for steroid stimulation of appetitive sexual behavior is limited. This is the overall objective of the proposed studies. Hamsters are an important model for investigating sexual motivation because mating is dependent on chemosensory cues. Male hamsters engage in extensive anogenital investigation of females before mating, and removal of the olfactory bulbs abolishes both copulation and preference for an estrous female. Studies in Specific Aim 1 will investigate the effects of castration and steroid replacement on detection of and responsiveness to female hamster vaginal secretion (FHVS) to determine if castrated males can detect FHVS, and the steroid signals that convey responsiveness to FHVS. Conditioned taste avoidance will be used to assess chemosensory detection (Aim 1A), while Aims 1B and 1C will evaluate chemosensory responsiveness using operant responding for FHVS. Specific Aim 2 will determine brain pathways for hormonal control of chemosensory responsiveness. Aim 2A will investigate the transduction of rewarding chemosensory cues in the olfactory mucosa and/or vomeronasal organ. Aims 2B and 2C will focus on central targets of the olfactory bulbs. The medial preoptic area (MPOA) is key to expression of consummatory sexual behavior. However, lesions of MPOA fail to prevent preference for an estrous female, suggesting that other brain regions contribute to steroidal effects on sexual motivation. The medial amygdaloid nucleus (Me) is a logical site in this regard. Me receives efferents from the olfactory bulbs, and has abundant steroid receptors. We have shown that testosterone in Me stimulates sexual activity in castrated males. Aims 2B and 2C will use lesions and hormone stimulation to determine the role of Me and MPOA in steroid facilitation of chemosensory responsiveness. These studies have relevance to understanding sexual motivation, and the significance of steroid hormones in appetitive sexual behavior.